.root{/*整个页面的根*/
  background: white;
      width: 900px;
    height: 900px;
    margin: 0px auto;
    position: relative;
  h1{
    font-size: 30px;
    color: #f00;
  }
  .navdiv{
    /*根下的导航条*/
    position: absolute;
    left: 50px;
  }
}
